Method of obtaining a yellow gold alloy deposition by galvanoplasty without using toxic materials

ABSTRACT

The invention relates to a method of galvanoplastic deposition of a gold alloy on an electrode dipped into a bath including metal gold in alkaline aurocyanide form, organometallic compounds, a wetting agent, a sequestering agent and free cyanide. According to the invention, the alloy metals are copper, in double copper and potassium cyanide form, and silver in cyanide form, allowing a mirror bright yellow gold alloy to be deposited on the electrode. 
     The invention concerns the field of galvanic depositions.

This application claims priority from European Patent Application No. 09173198.4 filed Oct. 15, 2009, the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to electrolytic deposition in the form of a thick gold alloy layer and the method of fabricating the same.

BACKGROUND OF THE INVENTION

In the field of decorative plating, methods are known for producing yellow coloured, electrolytic depositions of gold, with a grade equal to or more than 9 carats, which is ductile, with a thickness of 10 microns and with a high level of tarnish resistance. These depositions are obtained by electrolysis in an alkaline galvanic bath containing 0.1 to 3 g·l⁻¹ cadmium, in addition to gold and copper.

The depositions obtained by these known methods have, however, a cadmium content of between 1 and 10%. Cadmium facilitates deposition of thick layers, i.e. between 1 and 800 microns and provides a yellow coloured alloy while reducing the quantity of copper contained in the alloy. However, cadmium is extremely toxic and prohibited in some countries.

18 carat gold alloys without any cadmium, which contain copper and zinc, are also known. However, these depositions have an excessively pink hue (too rich in copper). Finally, these depositions have poor corrosion resistance which means they tarnish quickly.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome all or part of the aforementioned drawbacks, by providing a manufacturing method for depositing a thick, yellow coloured, gold alloy layer which has neither zinc nor cadmium as its main constituents.

The invention therefore relates to a method for the galvanoplastic deposition of a gold alloy on an electrode dipped in a bath that includes metal gold in the form of alkaline aurocyanide, organometallic compounds, a wetting agent, a sequestering agent and free cyanide, characterized in that the alloy metals are copper, in the form of double copper and potassium cyanide, and silver, in cyanide form, allowing a mirror-bright yellow gold alloy to be deposited on the electrode.

According to other advantageous features of the invention:

-   -   the bath includes 1 to 10 g·l⁻¹ gold metal in the form of         alkaline aurocyanide;     -   the bath includes 30 to 80 g·l⁻¹ copper metal in the form of         alkaline double cyanide;     -   the bath includes 10 mg·l⁻¹ to 1 g·l⁻¹ silver metal in complex         form;     -   the bath includes 15 to 35 g·l⁻¹ cyanide;     -   the wetting agent has a concentration of between 0.05 and 10         ml·l⁻¹     -   the wetting agent is chosen from among polyoxyalkenic, ether         phosphate, lauryl sulphate, dimethyldodecylamine-N-oxide,         dimethyl(dodecyl) ammonium propane sulfonate;     -   the bath includes a concentration of amine of between 0.01 and 5         ml·l⁻¹;     -   the bath includes a concentration of depolariser of between 0.1         mg·l⁻¹ and 20 mg·l⁻¹;     -   the bath includes phosphate, carbonate, citrate, sulphate,         tartrate, gluconate and/or phosphonate type conductive salts;     -   the temperature of the bath is kept between 50 and 80° C.;     -   the pH of the bath is kept between 8 and 12;     -   the method is performed with a current density of between 0.05         and 1.5 A·dm⁻²;     -   the bath respects a proportion of 9.08% gold, 90.85% copper and         0.07% silver.

The invention also concerns an electrolytic deposition in the form of a gold alloy obtained from a method according to any of the preceding claims, whose thickness is comprised between 1 and 800 microns and which includes copper, characterized in that it includes silver as the third main compound in a proportion of 75% gold, 21% copper and 4% silver to obtain a bright 3N colour.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention concerns an electrolytic deposition of a gold alloy with a 3N colour which, surprisingly, includes Au—Cu—Ag as its main compounds in proportions that are not known, to obtain the 3N colour, i.e. bright yellow.

In the example deposition above, there is a gold alloy, free of toxic metals or metalloids, and in particular free of cadmium, with a 3N yellow colour, a thickness of 200 microns, excellent brightness and with a very high level of resistance to wear and tarnishing.

This deposition is obtained by electrolysis in an electrolytic bath of the type:

-   -   Au: 5.5 g·l⁻¹;     -   Cu: 55 g·l⁻¹;     -   Ag: 40 mg·l⁻¹;     -   KCN: 26 g·l⁻¹;     -   pH: 10.5;     -   Temperature: 65° C.;     -   Current density: 0.3 A·dm⁻²;     -   Wetting agent: 0.05 ml·l⁻¹ NN_Dimethyldodecyl N-oxide;     -   Iminodiacetic: 20 g·l⁻¹;     -   Ethylenediamene: 0.5 ml·l⁻¹;     -   Gallium, selenium or tellurium: 10 mg·l⁻¹.

The electrolysis is preferably followed by a heat treatment at a temperature of between 200 and 450 degrees Celsius for 1 to 30 minutes in order to obtain a deposition of optimum quality.

These conditions provide a cathodic yield of 98 mg·A·min⁻¹ with a deposition speed of around 10 μm per hour in the case of the example.

Thus, surprisingly, the bath according to the invention provides a deposition in proportions of around 75% gold, 21% copper and 4% silver, corresponding to a 3N colour, 18 carat deposition, very different proportions from the usual electrolytic depositions for this colour, which tend to be depositions of around 75% gold, 12.5% copper and 12.5% silver.

The bath may also contain a brightener. This is preferably a butynediol derivative, a pyridinio-propanesulfonate or a mixture of the two, a tin salt, sulfonated castor oil, methylimidozole, dithiocarboxylic acid such as thiocarbamide, thiobarbituric acid, imidazolidinthion or thiomalic acid.

In these examples, the electrolytic bath is contained in a polypropylene or PVC bath holder with a heat insulating coating. The bath is heated using quartz, PTFE, porcelain or stabilised stainless steel thermo-plungers. Good cathodic rod movement and electrolyte flow must be maintained. The anodes are made of platinum plated titanium, stainless steel, ruthenium, iridium or alloys of the latter two.

Of course, the present invention is not limited to the illustrated example but is capable of various variants and alterations which will be clear to those skilled in the art. In particular, the bath may contain the following metals: Zr, Se, Te, Sb, Sn, Ga, As, Sr, Be, Bi in negligible quantities.

Moreover, the wetting agent may be of any type that can wet in an alkaline cyanide medium. 

1. A method for the galvanoplastic deposition of a gold alloy on an electrode dipped into a bath including gold metal in alkaline aurocyanide form, organometallic compounds, a wetting agent, a sequestering agent and free cyanide, the alloy metals being copper, in double copper and potassium cyanide form, and silver, in cyanide form, allowing a mirror-bright yellow gold alloy to be deposited on the electrode wherein the bath respects a proportion of 9.08% gold, 90.85% copper and 0.07% silver containing neither cadmium nor zinc.
 2. The method according to claim 1, wherein the bath includes 5.5 g·l⁻¹ of gold metal in alkaline aurocyanide form, 55 g·l⁻¹ of copper metal in double alkaline cyanide form and 40 mg·l⁻¹ of silver metal in complex form.
 3. The method according to claim 2, wherein the bath includes from 15 to 35 g·l⁻¹of cyanide.
 4. The method according to claim 1, wherein the wetting agent includes a concentration of between 0.05 and 10 ml·l⁻¹.
 5. The method according to claim 1, wherein the wetting agent is chosen from among poly-oxy-alkenic, ether phosphate, lauryl sulphate, dimethyldodecylamine-N-oxide, dimethyl(dodecyl) ammonium propane sulfonate types.
 6. The method according to claim 1, wherein the bath includes a concentration of amine of between 0.01 and 5 ml·l⁻¹.
 7. The method according to claim 1, wherein the bath includes a concentration of depolariser of between 0.1 mg·l⁻¹ to 20 mg·l⁻¹.
 8. The method according to claim 1, wherein the bath includes phosphate, carbonate, citrate, sulphate, tartrate, gluconate and/or phosphonate type conductive salts.
 9. The method according to claim 1, wherein the temperature of the bath is kept between 50 and 80° C.
 10. The method according to claim 1, wherein the pH of the bath is kept between 8 and
 12. 11. The method according to claim 1, wherein the method is performed at a current density of between 0.3 A·dm⁻².
 12. An electrolytic deposition in the form of a gold alloy obtained from a method according to claim 1, the thickness of which is between 1 and 800 microns and which includes copper, wherein it includes silver as the third main compound allowing a bright 3N colour to be obtained.
 13. An electrolytic deposition in the form of a gold, cupper and silver alloy wherein the deposition is made of 75% gold, 21% copper and 4% silver, allowing a bright 3N colour to be obtained. 